Fair exchange protocol plays an important role in electronic commerce in the case of exchanging digital contracts. Verifiably encrypted signatures provide an optimistic solution to these scenarios with an off-line trusted third party. In this paper, we propose an identity-based verifiably encrypted signature scheme. The scheme is non-interactive to generate verifiably encrypted signatures and t...

We analyze an optimistic contract signing protocol of Asokan, Shoup, and Waidner as a case study in the applicability of formal methods to verification of fair exchange protocols. After discussing the challenges involved in formalizing fairness, we use Mur , a finitestate analysis tool, to discover a weakness in the protocol that enables a malicious participant to produce inconsistent versions ...

Fairness is a broad concept, covering a range of qualifications such as impartiality, courtesy, equity, sportsmanship, etc. Here, we focus on fairness in exchanging (electronic) goods, stipulating that none of the partners can take an undue advantage over the other. When is an interaction between two or more people called fair? Let us proceed with a few examples which reflect fairness as it is ...

Formal validation of security protocols is of utmost importance before they gain market or academic acceptance. In particular, the results obtained from the formal validation of the improved Optimistic Fair Exchange Protocol based on Signature Policies (OFEPSP+) are presented. OFEPSP+ ensures that no party gains an unfair advantage over the other during the protocol execution, while substantial...

transition relations. In Proc. 16th Ann. IEEE Symp. on Logic in Computer Science, Boston, MA, June, pp. 51–58.IEEE Computer Society Press, Los Alamitos, CA. [77] Kremer, S. and Raskin, J.-F. (2002) Game analysis of abuse-free contract signing. In Proc. 15th IEEE Computer Security Foundations Workshop, Cape Breton, Nova Scotia, Canada,June. IEEE Computer Society Press, Los Alamitos, ...

Ambiguous Optimistic Fair Exchange (AOFE), introduced by Huang et al. in ASIACRYPT 2008, is an extension of OFE that enhances the fairness of the two communicating parties in the exchange of signatures. The first scheme was proven secure without random oracles while its partial signature contains dozens of group elements. Recently, interactive AOFE was introduced and the construction is more pr...

In this paper we propose an e-commerce protocol with the following features: (1) ensures true fair exchange, (2) does not require manual dispute resolution in case of unfair behavior by any party, (3) does not require the active involvement of a trusted third party, (4) allows the customer to verify that the product he is about to receive is the one he is paying for, and (5) can be used for the...

This paper addresses the problem of designing practical protocols for proving properties about encrypted data. To this end, it presents a variant of the new public key encryption of Cramer and Shoup based on Paillier’s decision composite residuosity assumption, along with efficient protocols for verifiable encryption and decryption of discrete logarithms (and more generally, of representations ...

The work in this thesis has been carried out at the centre for mathematics and computer science (CWI), under the auspices of the research school IPA (Institute for Programming research and Algorithmics). The research has been funded by the Dutch organisation for scientific research (NWO) in the context of the ACCOUNT project on accountability in electronic commerce protocols. Acknowledgements T...

We present a fair and optimistic [7, 8] quantum contract signing protocol between two clients that requires no communication with the third trusted party during the exchange phase. We discuss its fairness and show that it is possible to design such a protocol for which the probability of a dishonest client to cheat becomes negligible, and scales as N−1/2, where N is the number of messages excha...